In a TOF-MS, ions accelerated by an electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass to charge ratios according to the time of flight until they reach and are detected by a detector. Since the difference of the lengths of flight time of two ions having different mass to charge ratios is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the resolution of the mass to charge ratio of a TOF-MS. In many cases, however, it is difficult to incorporate a long straight path in a TOF-MS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length.
In the Japanese Unexamined Patent Publication No. H11-297267, an elliptic orbit is formed using plural toroidal type sector-formed electric fields, and the ions are guided to fly repeatedly on the elliptic orbit many times, whereby the effective flight length is elongated. In the Japanese Unexamined Patent Publication No. H11-135060, ions fly an “8” shaped orbit repeatedly. In these TOF-MSs, the length of flight time of ions from the time when they start the ion source and to the time when they arrive at and are detected by the ion detector is measured, where the ions fly the closed orbit a predetermined times between the ion source and the ion detector. The mass to charge ratios of the ions are calculated based on the lengths of the flight time. As the number of turns the ions fly the orbit is larger, the length of flight time is longer, so that the resolution of the mass to charge ratio becomes better by increasing the number of turns.
In an ideal TOF-MS, ions of the same mass to charge ratio start at the same starting point with the same initial energy, and arrive at the ion detector together at the same time. But in an actual TOF-MS, diversity in the initial kinetic energy of ions of the same mass to charge ratio, difference in the starting point, variation in the starting time (jitter), variation in the detection timing (jitter), fluctuation of the source voltage, etc. cause errors in the measured length of the flight time. Since these error-causing factors are unrelated to mass to charge ratio of ions, the length of flight time is not exactly the function of the mass to charge ratio, and the errors of the flight time cannot be eliminated or decreased by increasing the number of turns that the ions fly the loop orbit. This prevents improving the accuracy of the mass analysis in such type of TOF-MSs.